1. Field of the Invention
This invention relates generally to polymeric films and coatings. It relates particularly to irradiated, thermally cured polyimide films.
2. Description of Related Art
It has been known for a relatively long time that bulk properties of materials are not necessarily exhibited when reduced in size to the nano-size regime. At least one group of scientists has been researching the metallization of high-performance polymer films containing metallic nanoparticulates in the bulk of the film.
Providing reflective surfaces on exterior surfaces of polyimide films is known in the related art. U.S. Pat. No. 6,019,926 discusses a method of providing reflective, silvered polyimide films via in situ thermal reduction silver (I) complexes. Additionally, technology such as U.S. Pat. No. 5,520,960 relates to electrically conductive polyimides containing silver trifluoroacetylacetonate. U.S. Pat. No. 5,575,955 discusses an electrically conductive polyimide film containing gold (III) ions, composition, and process of making. Additionally, U.S. Pat. No. 5,677,418 shows a method of producing reflective self-metallizing polyimide films.
It should be noted that all these disclosures appear to relate to providing a reflective top surface on a polyimide film. This general technology is referred to as Self-Metallized Film Technology and typically produces a surface metallized flexible polymer film having tunable specular reflectivity and surface electrical conductivity. In fact, it is known that polymers can inhibit the aggregation of metal particles by surface modifications that alter the specific surface energy of the metallic particles and thus their attraction to each other. W. Caseri, “Nanocomposites of polymers and metals or semiconductors: Historical background and optical properties,” Macromol. Rapid Commun 21, 705-722 (2000).
In Self-Metallized Film Technology, a homogeneous solution containing a soluble metal complex in a polymer resin is cast as a thin film and then subjected to thermal curing. The cure process induces in situ metal ion reduction in the formation of reduced metal clusters that produce a conductive reflective metallic surface layer with additional nanometer-sized metal particulates embedded in the bulk of the film. As long as only a reflective/conductive metallic surface layer is desired, this method is believed to provide a satisfactory method of providing a reflective/conductive metallic surface layer on the polyimide film. Unfortunately, during this process, the metal particulates imbedded within the bulk of the film are dispersed in density gradients of dispersed metallic particulates characterized by non-uniformity in the size of these nanometer-size particles. No continuous subsurface or interlayers are created.
Apparently some work has focused on electroless deposition of silvery interlayers within polymer films. Specifically, L. E. Manring prepared an article entitled “Electroless Deposition of Silver as an Interlayer Within Polymer Films”, Polymer Communications, 1987, Volume 28, March, pp. 68-71. This document discusses forming a metal layer as an interlayer within polymeric films using counter-current diffusion as opposed to the old method of electroless-deposition. The Journal of Physical Chemistry provided an article in 1987 entitled “The Kinetics of Metal Interlayer Growth in Polyimide Films: Metal Distributions in the Non-Shady-Sate Regime and with Constraints of Patterned Boundaries.” J. Phys. Chem., 1987, 91, 6699-6705. This article defined “interlayer” as being known in the art to distinguish the structure from conventional surface-metallized films. The metal in the interlayer in this reference is precipitated from the reaction of dissolved metal ions either with a reducing agent or with mobile electrons. These two components (reducing agent or mobile electrons) are introduced into the film from opposite surfaces. It is believed that the transport of reagents governs the location of the inner layer. This article discusses how one might control the reaction/precipitation process to predict and control electrical and optical properties of the final film product.
These methods of achieving an embedded, or sub-surface, metallic layer involve counter-current diffusion in free standing films. These processes do not typically result in an additional well-adhered surface metallic layer. Furthermore, these processes typically do not provide uniform size and distribution of metallic nanoparticulates in the bulk of the film above and below the interlayer. Finally, films formed by these processes have not been reported to exhibit dimensional changes upon exposure to white light, and the optical properties of these films were not reported in the known references.
While the use of electrical and chemical processes including the use of a reducing agent with mobile electrons is known in the art to produce an interlayer, there is believed to be a need for an improved method of providing an interlayer within polymeric films.